This invention relates to internal combustion engines, and more particularly to the improvements in internal combustion engines for use in automobiles.
During idle running and low-load running of an internal combustion engine for use in an automobile, the throttle valve is opened only a small degree and hence the amount of intake air is small. Accordingly, the rate at which a charge mixture is introduced through the intake manifold into the cylinder during the suction stroke is low, resulting in a weak swirl of the charge mixture within the cylinder. This also weakens the swirl of the mixture charge remaining in the cylinder at the time of ignition, which substantially takes place at the terminal stage of a compression stroke, resulting in lowered ingitability and combustibility. In order to insure the stable running of an engine in such running conditions, a charge mixture whose air-fuel ratio is lower than that for medium and high-load running must be provided. However, this causes increased fuel consumption, as well as an increase in the quantity of harmful constituents, such as CO and HC, contained in exhaust gases due to incomplete combustion of a rich charge mixture.
Recently, with a view to reducing the quantity of harmful constituents, such as CO and HC and particularly NO.sub.x, contained in exhaust gases from an engine, it has been proposed to burn a charge mixture much leaner than the theoretical or stoichiometric air-fuel ratio. Also, with a view to reducing the amount of NO.sub.x, it has been proposed to take a part of the exhaust gases from the exhaust system and mix it with a charge mixture for combustion. Both proposals result in lowered ignitability as well as lowered combustibility of the charge mixture, resulting in lowered drivability at idling and low-load conditions as well as increased fuel consumption.